Balance circuits for battery cells

ABSTRACT

A balance circuit for a set of battery cells includes a set of switch circuits and control circuitry coupled to the switch circuits. Each switch circuit of the switch circuits is coupled to a corresponding battery cell of the battery cells, and enables a bypass current to flow out a positive terminal of the corresponding battery cell if the switch circuit is turned on. The switch circuit includes a first switch having a first diode and a second switch having a second diode reversely coupled to the first diode. The second switch disables the bypass current if the second switch is turned off The control circuitry balances the battery cells by controlling the switch circuits.

RELATED APPLICATION

This application claims benefit under 35 U.S.C. § 119(a) to Application No. GB1805881.8, filed with the United Kingdom Intellectual Property Office on Apr. 9, 2018, hereby incorporated herein by reference in its entirety.

BACKGROUND

FIG. 1A illustrates a battery pack 100 including a conventional balance circuit 112 for a set of battery cells 110. The balance circuit 112 includes current limit resistors R₀, R₁, R₂, R₃ and R_(4,) switches S₁, S₂, S₃ and S₄, and a controller 102. The controller 102 monitors cells voltages of the battery cells 110 and balances the battery cells 110 based on the monitored information. For example, during a charging process, if a battery cell Cell_(M) (M=1, 2, 3 or 4) of the battery cells 110 has a cell voltage greater than a balance threshold, then the controller 102 turns on a corresponding switch S_(M) (M=1, 2, 3 or 4) to bypass a portion I_(PT) of the charging current of the battery cell Cell_(M), and therefore the rate of increase of the cell voltage of the battery cell Cell_(M) is lower than that of the other battery cells. As another example, if a battery cell Cell_(N) (N=1, 2, 3 or 4) has a cell voltage greater than the cell voltages of the other battery cells by an amount that exceeds a voltage reference, then the controller 102 turns on a corresponding switch S_(N) (N=1, 2, 3 or 4) to bypass a portion I_(PT) of the charging current of the battery cell Cell_(N). As a result, differences between the cell voltages of the battery cells 110 are reduced, and therefore the battery cells 110 are balanced.

However, the conventional balance circuit 112 has some shortcomings. For example, if a battery cell Cell_(X) (X=1, 2, 3 or 4) of the battery cells 110 is reversely connected to the other battery cells, then a body diode of the switch S_(X) (X=1, 2, 3 or 4) is turned on to cause a leakage current I_(RV) to discharge the battery cell Cell_(X). The leakage current I_(RV) flowing through the body diode may generate enough heat to damage the balance circuit 112 and/or damage the integrated circuit (IC) package that includes the balance circuit 112. As another example, if a battery cell, e.g., Cell₄, of the battery cells 110 is disconnected from the balance circuit 112, then a body diode of the switch S₄ is turned on, which causes a leakage current to flow from the battery cells Cell₁, Cell₂ and Cell₃, through the body diode of the switch S₄, to charge a filter capacitor C_(C) that is coupled to a positive terminal PACK+ of the battery pack 100. That leakage current is relatively large and may damage the switch S₄.

More specifically, FIG. 1B illustrates a battery pack 100A including the conventional balance circuit 112, in which a battery cell Cell₂ of the battery cells 110 is reversely connected to the other battery cells. As shown in FIG. 1B, the reversely connected battery cell Cell₂ applies a forward-bias voltage to the body diode of the switch S₂, and therefore the body diode of the switch S₂ is turned on to discharge the battery cell Cell₂. The leakage current I_(RV) of the battery cell Cell₂ not only may over-discharge the battery cell Cell₂ but also may generate enough heat to damage the IC package.

FIG. 1C illustrates a situation in which a battery cell Cell₄ of the battery cells 110 is disconnected from the conventional balance circuit 112. For example, during assembly of the battery pack 100B, the battery cells Cell₁, Cell₂, Cell₃ and Cell₄ are supposed to be connected to (e.g., welded to) the balance circuit 112, one by one, e.g., from the bottom to the top. However, if the battery cells Cell₁, Cell₂ and Cell₃ are connected to the switches S₁, S₂, and S₃, but the battery cell Cell₄ is not connected to the switch S₄, a voltage V_(C) across the series-coupled battery cells Celli, Cell₂ and Cell₃ will be applied to the switch S₄, the resistor R₄, and the capacitor C_(C). If the voltage V_(C) is relatively large, the body diode of the switch S₄ will be forward biased, e.g., turned on, and a leakage current I_(LK) flowing from the battery cells Cell₁, Cell₂ and Cell₃, through the body diode of the switch S₄ and the resistor R₄, to charge the capacitor C_(C) will also be relatively large. A relatively large leakage current I_(LK) may cause damage to the switch S₄. As another example, when the battery pack 100B is finally assembled into a package, one or more of the battery cells 110 may have a loose connection with the balance circuit 112. If a battery cell, e.g., Cell₃, has a loose connection with the balance circuit 112, then a voltage across the series-coupled battery cells Cell₁ and Cell₂ will be applied to the switches S₃ and S₄, the resistor R₄, and the capacitor C_(C). Similar to the other example, the body diodes of the switches S₃ and S₄ will be turned on and a relatively large leakage current will flow through the body diodes to charge the capacitor C_(C), which may cause damage to the switches S₃ and S₄.

Thus, a balance circuit that addresses the abovementioned shortcomings would be beneficial.

SUMMARY

In an embodiment, a balance circuit for a set of battery cells includes a set of switch circuits and control circuitry coupled to the switch circuits. Each switch circuit of the switch circuits is coupled to a corresponding battery cell of the battery cells, and can enable a bypass current to flow from a positive terminal of the corresponding battery cell if the switch circuit is turned on. The switch circuit includes a first switch having a first diode, and also includes a second switch having a second diode reversely coupled to the first diode. The second switch can disable the bypass current if the second switch is turned off The control circuitry can balance the battery cells by controlling the switch circuits.

BRIEF DESCRIPTION OF THE DRAWINGS

Features and advantages of embodiments of the claimed subject matter will become apparent as the following detailed description proceeds, and upon reference to the following drawings, wherein like numerals depict like parts.

FIG. 1A illustrates a battery pack including a conventional balance circuit for a set of battery cells.

FIG. 1B illustrates a battery pack including a conventional balance circuit, in which a battery cell of a set of battery cells is reversely connected to the other battery cells.

FIG. 1C illustrates a situation in which a battery cell of a set of battery cells is disconnected from a conventional balance circuit.

FIG. 2A illustrates a circuit diagram of an example of a balance circuit for a set of battery cells in a battery pack, in an embodiment of the present invention.

FIG. 2B illustrates a circuit diagram of an example of a balance circuit for a set of battery cells in a battery pack, in an embodiment of the present invention.

FIG. 2C illustrates a circuit diagram of an example of a balance circuit for a set of battery cells in a battery pack, in an embodiment of the present invention.

FIG. 2D illustrates a circuit diagram of an example of a balance circuit for a set of battery cells in a battery pack, in an embodiment of the present invention.

FIG. 3A illustrates a circuit diagram of an example of a balance circuit for a set of battery cells in a battery pack, in an embodiment of the present invention.

FIG. 3B illustrates a circuit diagram of an example of a balance circuit for a set of battery cells in a battery pack, in an embodiment of the present invention.

FIG. 4 illustrates a flowchart of examples of operations performed by a balance circuit for a set of battery cells, in an embodiment of the present invention.

DETAILED DESCRIPTION

Reference will now be made in detail to the embodiments of the present invention. While the invention will be described in conjunction with these embodiments, it will be understood that they are not intended to limit the invention to these embodiments. On the contrary, the invention is intended to cover alternatives, modifications and equivalents, which may be included within the spirit and scope of the invention as defined by the appended claims.

Furthermore, in the following detailed description of the present invention, numerous specific details are set forth in order to provide a thorough understanding of the present invention. However, it will be recognized by one of ordinary skill in the art that the present invention may be practiced without these specific details. In other instances, well known methods, procedures, components, and circuits have not been described in detail as not to unnecessarily obscure aspects of the present invention.

FIG. 2A illustrates a circuit diagram of an example of a balance circuit 212A for a set of battery cells 220 in a battery pack 200A, in an embodiment of the present invention. In an embodiment, the battery cells 220 are coupled in series. Although FIG. 2A shows four battery cells in the battery pack 200A, the invention is not so limited. In another embodiment, the battery pack 200A can include another number, e.g., 2, 3, 5, 6 or 7, of battery cells. In an embodiment, the battery cells 220 include rechargeable battery cells such as lithium-ion battery cells. In other embodiments, the battery cells 220 may include nickel-cadmium battery cells, lead-acid battery cells, solar battery cells, or the like.

In an embodiment, the balance circuit 212A includes a set of switch circuits and control circuitry 202A. Each switch circuit is coupled to a corresponding battery cell of the battery cells 220, and can enable a bypass current to flow from a positive terminal of the corresponding battery cell if the switch circuit is turned on. Additionally, each switch circuit includes a first switch having a first body diode, and also includes a second switch having a second body diode reversely coupled to the first body diode. If the second switch if turned off, the second switch can disable the bypass current. As used herein, “a second diode is reversely coupled to a first diode” means that either both the cathodes of the diodes are coupled to a connection node between the cathodes or both the anodes of the diodes are coupled to a connection node between the anodes.

For example, as shown in FIG. 2A, the switch circuit coupled to the battery cell Cell₁ (hereinafter, switch circuit Q_(N11)-Q_(N12)) includes a first switch Q_(N11) having a first body diode, and also includes a second switch Q_(N12) having a second body diode. In an embodiment, the anode of the first body diode and the anode of the second body diode are coupled to a connection node 222 between the two anodes (between the first and second body diodes). This kind of switch circuit may be referred to as a “back-to-back switch.” If the first switch Q_(N11) and the second switch Q_(N12) are turned on, then the switch circuit Q_(N11)-Q_(N12) is turned on, and a bypass current I_(BY) can be enabled to flow from a positive terminal 228 of the battery cell Cell₁, through the switch circuit Q_(N11)-Q_(N12), to a negative terminal 226 of the battery cell Cell₁. In a charging process of the battery cells 220, the bypass current I_(BY) can reduce the rate of increase of a cell voltage V_(CELL1) of the battery cell Cell₁. If the battery cells 220 are neither charging nor discharging, then the bypass current I_(BY) can discharge the battery cell Cell₁ to reduce the cell voltage V_(CELL1). If the second switch Q_(N12) is turned off, then the second switch Q_(N12) can disable/block the bypass current I_(BY). Similarly, if the first switch Q_(N11) is turned off, then the first switch Q_(N11) can disable/block a current flowing from the terminal 226 to the terminal 228. If the first switch Q_(N11) and the second switch Q_(N12) are turned off, then no current flows through the switch circuit Q_(N11)-Q_(N12). In an embodiment, the circuit structures and functions of the switch circuits Q_(N21)-Q_(N22), Q_(N31)-Q_(N32), and Q_(N41)-Q_(N42), coupled to the battery cells Cell₂, Cell₃ and Cell₄ respectively, are similar to that of the switch circuit Q_(N11)-Q_(N12). The switches Q_(N21), Q_(N31) and Q_(N41) can be referred to as the first switches of the switch circuits Q_(N21)-Q_(N22), Q_(N31)-Q_(N32), and Q_(N41)-Q_(N42), respectively. The switches Q_(N22), Q_(N32) and Q_(N42) can be referred to as the second switches of the switch circuits Q_(N21)-Q_(N22), Q_(N31)-Q_(N32), and Q_(N41)-Q_(N42), respectively.

Accordingly, in the example of FIG. 2A, if a battery cell Cell_(K) (e.g., K=1, 2, 3 or 4) is reversely connected to the other battery cells, then a leakage current (e.g., similar to the leakage current IRv mentioned in relation to FIG. 1B) of the battery cell Cell_(K) can be blocked/disabled by turning off the first switch (e.g., Q_(N11), Q_(N21), Q_(N31) and Q_(N41)) of the corresponding switch circuit. For example, if the battery cell Cell₂ is reversely connected to the battery cells Cell₁, Cell₃ and Cell₄, then turning off the switch Q_(N21) can disable/block a leakage current of the battery cell Cell₂. Because all the switches Q_(N11), Q_(N12), Q_(N21), Q_(N22), Q_(N31), Q_(N32), Q_(N41) and Q_(N42) are initially off, the balance circuit 212A and/or the IC package that includes the balance circuit 212A can be protected from damage by the leakage current I_(RV).

Additionally, in an embodiment, the battery pack 200A includes a capacitor C_(C) coupled to a positive terminal PACK+ of the battery pack 200A. The capacitor C_(C) can filter out, e.g., voltage spikes and/or current spikes, at the terminal PACK+. In an embodiment, if a top battery cell, e.g., Cell₄, of the battery cells 220 is disconnected from or has a loose connection with a top switch circuit, e.g., Q_(N41)-Q_(N42), of the switch circuits, then a current flowing from a battery cell Cell₃ that is adjacent to the top battery cell Cell₄ to charge the capacitor C_(C) can be blocked because the top switch circuit Q_(N41)-Q_(N42) is initially turned off. As a result, the balance circuit 212A can protect the switch circuit Q_(N41)-Q_(N42) from being damaged by a relatively large leakage current, e.g., similar to the leakage current I_(LK) mentioned in relation to FIG. 1C, from the battery cells Cell₁, Cell₂ and Cell₃. Similarly, in an embodiment, if a battery cell below the top battery cell (e.g., the battery cell Cell₃) is disconnected from or has a loose connection with the switch circuit Q_(N31)-Q_(N32), then a current flowing from the battery cell Cell₂ to charge capacitor C_(C) can be blocked because the switch circuit Q_(N31)-Q_(N32) and/or the switch circuit Q_(N41)-Q_(N42) are initially turned off Thus, the balance circuit 212A can protect the switch circuits Q_(N31)-Q_(N32) and Q_(N41)-Q_(N42).

Moreover, in an embodiment, the control circuitry 202A can balance the battery cells 220 by controlling, e.g., selectively turning on or off, the switch circuits Q_(N11)-Q_(N12), Q_(N21)-Q_(N22), Q_(N31)-Q_(N32), and Q_(N41)-Q_(N4) 2, thereby extending the battery life of the battery cells. For example, the control circuitry 202A can monitor a status, e.g., cell voltages, of the battery cells 220. If the control circuitry 202A detects that a battery cell Cell_(Q) (e.g., Q=1, 2, 3 or 4) of the battery cells 220 has a cell voltage greater than a balance reference, then the control circuitry 202A turns on a corresponding switch circuit of the switch circuits by turning on the first switch and the second switch in the corresponding switch circuit. In an embodiment, the balance reference is a preset voltage reference. In another embodiment, the balance reference is determined by a minimum cell voltage of the cell voltages of the battery cells 220. For example, the balance reference can be equal to the minimum cell voltage plus a preset voltage. In yet another embodiment, the balance reference is determined by an average voltage of the cell voltages of the battery cells 220. As a result, the battery cells 220 can be balanced.

In an embodiment, the switches Q_(N11), Q_(N12), Q_(N21), Q_(N22), Q_(N31), Q_(N32), Q_(N41) and Q_(N42) of the switch circuits are metal-oxide-semiconductor field-effect transistors (MOSFETs). In one such embodiment, the connection node 222 between the anode of the body diode of the first MOSFET Q_(N11) and the anode of the body diode of the second MOSFET Q_(N12) includes a connection node between a source of the first MOSFET Q_(N11) and a source of the second MOSFET Q_(N12).

In the example of FIG. 2A, anodes of the body diodes of each switch circuit of the switch circuits are coupled to a corresponding connection node. However, the invention is not so limited. In other embodiments, e.g., as shown in FIG. 2B and FIG. 2D, cathodes of the body diodes of each switch circuit of the switch circuits are coupled to a corresponding connection node between the cathodes. For example, in FIG. 2B, the cathode of the body diode of the first switch Q_(N11) and the cathode of the body diode of the second switch Q_(N12) are coupled to a connection node 224 between the cathodes.

Additionally, in the example of FIG. 2A, the switches in the switch circuits include re-channel MOSFETs. However, the invention is not so limited. In other embodiments, the switches in the switch circuits include p-channel MOSFETs, e.g., as shown in FIG. 2C and FIG. 2D.

In the examples of FIG. 2A, FIG. 2B, FIG. 2C and FIG. 2D, each switch circuit of the switch circuits includes a first MOSFET and a second MOSFET. The switch circuit can be turned on by turning on both the MOSFETs, and power consumption of the switch circuit is relatively small due to the low-power consumption feature of MOSFETs. However, the invention is not so limited. In other embodiments, the switch circuit includes a transistor such as a MOSFET and a PN junction diode.

By way of example, as shown in FIG. 3A, the switch circuit (hereinafter, switch circuit Q_(N12)-D₁₁) coupled to the battery cell Celli includes a transistor Q_(N12) and a diode D₁₁, e.g., a PN junction diode. In an embodiment, the transistor Q_(N12) can function as a switch under control of the control circuitry 302A, and the diode D₁₁ can function as a switch under control of the transistor Q_(N12). In the example of FIG. 3A, the transistor Q_(N12) includes an n-channel MOSFET. Thus, the control circuitry 302A can control the driver circuit 304A to pull up a voltage at the gate terminal of the transistor Q_(N12) to turn on the transistor Q_(N12), or to pull down the voltage at the gate terminal of the transistor Q_(N12) to turn off the transistor Q_(N12). If the transistor Q_(N12) is turned on, then the battery cell Cell₁ can apply a forward bias voltage to the diode D₁₁, through the resistors R₀ and R₁ and the transistor Q_(N12), to turn on the diode D₁₁. The switch circuit Q_(N12)-D₁₁ can also be turned on. If the transistor Q_(N12) is turned off, then the diode D₁₁, as well as the switch circuit Q_(N12)-D₁₁, can be turned off.

The diode D₁₁ can be referred to as a first switch of the switch circuit Q_(N12)-D₁₁, and the transistor Q_(N12) can be referred to as a second switch of the switch circuit Q_(N12)-D₁₁. In an embodiment, the circuit structures and functions of the switch circuits Q_(N22)-D₂₁, Q_(N32)-D₃₁, and Q_(N42)-D₄₁, coupled to the battery cells Celle, Cell₃ and Cell₄ respectively, are similar to that of the switch circuit Q_(N12)-D₁₁. The diodes D₂₁, D₃₁ and D₄₁ can be referred to as the first switches of the switch circuits Q_(N22)-D₂₁, Q_(N32)-D₃₁, and Q_(N42)-D₄₁, respectively. The transistors Q_(N22), Q_(N32) and Q_(N42) can be referred to as the second switches of the switch circuits Q_(N22)-D₂₁, Q_(N32)-D₃₁, and Q_(N42)-D₄₁, respectively.

In an embodiment, the diodes D₁₁, D₂₁, D₃₁ and D₄₁ are unidirectional conducting devices, and they block currents flowing from their cathodes to their anodes. Thus, if a battery cell Cell_(K) (e.g., K=1, 2, 3 or 4) is reversely connected to the other battery cells, the diode D_(K1) can block a leakage current, e.g., similar to the leakage current I_(RV) mentioned in relation to FIG. 1B, flowing from the battery cell Cell_(K). Thus, the balance circuit 312A and/or the IC package that includes the balance circuit 312A can be protected from being damaged by the leakage current I_(RV) mentioned in relation to FIG. 1B.

Additionally, in an embodiment, if the top battery cell Cell₄ of the battery cells 220 is disconnected from or has a loose connection with the top switch circuit Q_(N42)-D₄₁, the diode D₄₁ can block a leakage current, e.g., similar to the leakage current I_(LK) mentioned in relation to FIG. 1C, flowing from the battery cells Cell₁, Cell₂ and Cell₃ to the capacitor C_(C). Thus, the switch circuit Q_(N42)-D₄₁ can be protected from being damaged by the leakage current I_(LK). Similarly, if a battery cell below the top cell (e.g., the battery cell Cell₃) is disconnected from or has a loose connection with the switch circuit Q_(N32)-D₃₁, the diode D₃₁ and/or the diode D₄₁ can block a leakage current flowing from the battery cells Cell₁ and Cell₂ to the capacitor C_(C). Thus, the switch circuits Q_(N32)-D₃₁ and Q_(N42)-D₄₁ can be protected from being damaged by the leakage current.

In the example of FIG. 3A, each of the switch circuits includes an n-channel MOSFET and a diode. However, the invention is not so limited. In another embodiment, each of the switch circuits includes a p-channel MOSFET and a diode, e.g., as shown in FIG. 3B. Operations and functions of the balance circuit 312B in FIG. 3B are similar to that of the balance circuit 312A in FIG. 3A.

FIG. 4 illustrates a flowchart 400 of examples of operations performed by a balance circuit for a set of battery cells, in an embodiment of the present invention. FIG. 4 is described in combination with FIG. 2A, FIG. 2B, FIG. 2C, FIG. 2D, FIG. 3A and FIG. 3B. Although specific steps are disclosed in FIG. 4, such steps are examples for illustrative purposes. That is, embodiments according to the present invention are well suited to performing various other steps or variations of the steps recited in FIG. 4.

At step 402, the control circuitry (e.g., 202A, 202B, 202C, 202D, 302A or 302B) balances a set of battery cells 220 by controlling a set of switch circuits (e.g., Q_(N11)-Q_(N12), Q_(N21)-Q_(N22), Q_(N31)-Q_(N32), and Q_(N41)-Q_(N42) in FIG. 2A; Q_(N12)-Q_(N11), Q_(N22)-Q_(N21), Q_(N32)-Q_(N31), and Q_(N42)-Q_(N41) in FIG. 2B; Q_(P11)-Q_(P12), Q_(P21)-Q_(P22), Q_(P31)-Q_(P32), and Q_(P41)-Q_(P42) in FIG. 2C; Q_(P12)-Q_(P11), Q_(P22)-Q_(P21), Q_(P32)-Q_(P31), and Q_(P42)-Q_(P41) in FIG. 2D; Q_(N12)-D₁₁, Q_(N22)-D₂₁, Q_(N32)-D₃₁, and Q_(N42)-D₄₁ in FIG. 3A; or D₁₁-Q_(P12), D₂₁-Q_(P22), D₃₁-Q_(P32), and D₄₁-Q_(P42) in FIG. 3B). Each switch circuit of the switch circuits is coupled to a corresponding battery cell of the battery cells. In an embodiment, each switch circuit includes a first switch having a first diode, and also includes a second switch having a second diode reversely coupled to the first diode. In an embodiment, the control circuitry balances the battery cells 220 by enabling or disabling a bypass current of a battery cell of the battery cells 220.

For example, at step 404, the control circuitry, e.g., 202A in FIG. 2A, can enable a bypass current IBY to flow from a positive terminal of the battery cell Cell₁ by turning on the switch circuit Q_(N11)-Q_(N12).

At step 406, the control circuitry 202A can disable the bypass current I_(BY) by turning off the second switch Q_(N11) or turning off the switches Q_(N11) and Q_(N12).

While the foregoing description and drawings represent embodiments of the present invention, it will be understood that various additions, modifications and substitutions may be made therein without departing from the spirit and scope of the principles of the present invention as defined in the accompanying claims. One skilled in the art will appreciate that the invention may be used with many modifications of form, structure, arrangement, proportions, materials, elements, and components and otherwise, used in the practice of the invention, which are particularly adapted to specific environments and operative requirements without departing from the principles of the present invention. The presently disclosed embodiments are therefore to be considered in all respects as illustrative and not restrictive, the scope of the invention being indicated by the appended claims and their legal equivalents, and not limited to the foregoing description. 

We claim:
 1. A balance circuit for a plurality of battery cells, said balance circuit comprising: a plurality of switch circuits comprising a switch circuit coupled to a corresponding battery cell of said plurality of battery cells, said switch circuit operable for enabling a bypass current to flow from a positive terminal of said corresponding battery cell if said switch circuit is turned on, wherein said switch circuit comprises a first switch having a first diode and a second switch having a second diode reversely coupled to said first diode, and wherein said second switch is configured to disable said bypass current if said second switch is turned off; and control circuitry, coupled to said switch circuits, operable for balancing said battery cells by controlling said switch circuits.
 2. The balance circuit of claim 1, wherein a first anode of said first diode and a second anode of said second diode are coupled to a connection node between said first anode and said second anode.
 3. The balance circuit of claim 2, wherein said first switch comprises a first metal-oxide-semiconductor field-effect transistor (MOSFET), said second switch comprises a second MOSFET, and said connection node comprises a connection node between a source of said first MOSFET and a source of said second MOSFET.
 4. The balance circuit of claim 1, wherein a first cathode of said first diode and a second cathode of said second diode are coupled to a connection node between said first cathode and said second cathode.
 5. The balance circuit of claim 1, wherein said control circuitry is configured to monitor a status of said battery cells, and wherein if said control circuitry detects that said corresponding battery cell has a cell voltage greater than a balance reference, then said control circuitry is configured to turn on said switch circuit by turning on said first switch and said second switch.
 6. The balance circuit of claim 1, wherein said battery cells are coupled in series.
 7. A method comprising: balancing a plurality of battery cells by controlling a plurality of switch circuits, wherein a switch circuit of said plurality of switch circuits is coupled to a corresponding battery cell of said battery cells, wherein said switch circuit comprises a first switch having a first diode and also comprises a second switch having a second diode reversely coupled to said first diode, and wherein said balancing comprises: enabling a bypass current to flow from a positive terminal of said corresponding battery cell by turning on said switch circuit; and disabling said bypass current by turning off said second switch.
 8. The method of claim 7, wherein a first anode of said first diode and a second anode of said second diode are coupled to a connection node between said first anode and said second anode.
 9. The method of claim 8, wherein said first switch comprises a first metal-oxide-semiconductor field-effect transistor (MOSFET), said second switch comprises a second MOSFET, and said connection node comprises a connection node between a source of said first MOSFET and a source of said second MOSFET.
 10. The method of claim 7, wherein a first cathode of said first diode and a second cathode of said second diode are coupled to a connection node between said first cathode and said second cathode.
 11. The method of claim 7, further comprising: monitoring a status of said battery cells; and if said corresponding battery cell has a cell voltage greater than a balance reference, then turning on said switch circuit by turning on said first switch and said second switch.
 12. The method of claim 7, wherein said battery cells are coupled in series.
 13. The method of claim 12, wherein said battery cells are included in a battery pack having a capacitor coupled to a positive terminal of said battery pack, wherein said battery cells comprise a first battery cell and a second battery cell adjacent to said first battery cell, wherein said switch circuits comprise a first switch circuit coupled to said first battery cell, and wherein said method further comprises: blocking a current from said second battery cell to said capacitor by turning off said first switch circuit.
 14. A battery pack comprising: a plurality of battery cells; a plurality of switch circuits, wherein a switch circuit of said switch circuits is coupled to a corresponding battery cell of said battery cells and is operable for enabling a bypass current to flow from a positive terminal of said corresponding battery cell if said switch circuit is turned on, wherein said switch circuit comprises a first switch having a first diode and also comprises a second switch having a second diode reversely coupled to said first diode, and wherein said second switch disables said bypass current if said second switch is turned off; and control circuitry, coupled to said switch circuits, operable for balancing said battery cells by controlling said switch circuits.
 15. The battery pack of claim 14, wherein a first anode of said first diode and a second anode of said second diode are coupled to a connection node between said first anode and said second anode.
 16. The battery pack of claim 15, wherein said first switch comprises a first metal-oxide-semiconductor field-effect transistor (MOSFET), said second switch comprises a second MOSFET, and said connection node comprises a connection node between a source of said first MOSFET and a source of said second MOSFET.
 17. The battery pack of claim 14, wherein a first cathode of said first diode and a second cathode of said second diode are coupled to a connection node between said first cathode and said second cathode.
 18. The battery pack of claim 14, wherein said control circuitry monitors a status of said battery cells, and wherein if said control circuitry detects that said corresponding battery cell has a cell voltage greater than a balance reference, then said control circuitry turns on said switch circuit by turning on said first switch and said second switch.
 19. The battery pack of claim 14, wherein said battery cells are coupled in series.
 20. The battery pack of claim 19, further comprising a capacitor coupled to a positive terminal of said battery pack, wherein said battery cells comprise a first battery cell and a second battery cell adjacent to said first battery cell, and wherein said switch circuits comprise a first switch circuit, coupled to said first battery cell, and operable for blocking a current from said second battery cell to said capacitor if said first switch circuit is turned off. 